ZIP files of ready-to-install CTAN packages. The package contained in
CTAN directory a/b/c/d/ appears as
install/a/b/c/d.tds.zip – each such file may be unzipped
directly into
a TDS
tree, and should contain no “tree prefix”

Material which is now
obsolete, but which needs to remain on the archive for some reason (such
as the 1992 LaTeX 2.09
distribution). The tree maintains a “shadow hierarchy”
– for example obsolete/macros
has the same basic structures macros.